This invention relates to electromagnetic wave generators for achieving very high frequency and high power. More particularly, it relates to a gyrotron device for generating a linear beam of electrons. Gyrotron devices are frequently used for injecting a high power electromagnetic wave into a nuclear fusion reactor in order to heat the plasma to reach the fusion ignition temperature.
FIG. 1 shows a well known gyrotron device. The gyrotron device comprises an electron gun 10 for emitting an electron beam in the direction of a dielectric window 51, a magnetic coil 22 for giving a cyclotron movement to the electrons, a resonator 30 for resonating the electromagnetic wave generated from the electron beam, and an output section for transmitting the electromagnetic wave through the window 51. The electron gun 10 comprises a conically tapered thermionic cathode 11 with an emitter 14 which is heated by a radiant internal heater 15. Surrounding cathode 11 is control electrode 12, and surrounding electrode 12 is solenoid 21 which produces an axial magnetic field. The heater 15 is energized by a power supply 17 through a terminal lead 16. Cathode 11 is connected to a D.C. power supply 80 which comprises a D.C. voltage generator 81 and dividing resistors 82, 83. The divided voltage is supplied to the control electrode 12. Each of the electrodes 11, 12, and 13 are insulated by insulators 71, 72.
Electron gun 10 emits a hollow electron beam which is drawn by a positive potential into the main body 20 of the gyrotron. The electrons travel in a helical pattern, thus creating a beam which rotates about its own axis as it travels in the gyrotron. An axial magnetic field formed by the magnetic coil 22 increases greatly as the electron stream passes into the entrance area of anode 13. The electron beam is compressed in diameter due to the effects of this magnetic field. Also, the speed of rotation about its axis is increased while its axial velocity is decreased. Axial energy is thereby converted into rotational energy. After the beam is compressed, it enters the interaction cavity 30. This is a circularly symmetric cavity with high-conductivity walls of copper. The cavity 30 is dimensioned to be electro-magnetically reasonant in a mode with the circular electric field perpendicular to its axis. At the beam input end, the wall of cavity 30 is constricted to form an aperture of a diameter small enough to prevent transmission of the cavity wave with consequent loss of energy. At the beam output end a similar aperture (second neck portion) is not completely cut off for the wave, but allows the desired fraction to emerge through the dielectric vacuum window 51 to enter a useful load (not shown) connected to an output micro-wave guide 52. Interaction cavity 30 is tapered larger in diameter toward its output end so that the amplitude of the standing wave increase for cumulative interaction.
After leaving cavity 30, the beam enters a region of decreasing magnetic field strength and its diameter increases accordingly until it is collected on the outer wall of propagating waveguide 42 which is cooled by water channels 44. Thus the functions of beam collector and output waveguide are combined.
In order to obtain a high oscillation efficiency, the ratio of the orbital velocity of the electron beam (V.sub.0) to the longitudinal velocity (V.sub.1) must be high. This ratio is know as the pitch factor. When the pitch factor is high, however, a large starting current must be used, which requires a large power source. If the size of the power source is limited, a lower pitch factor must be used which will result in a reduced oscillation efficiency.